Generally, a hot rolling apparatus for fabricating seamless steel pipes comprises a pair of upper and lower tapered rolls of a barrel shape disposed in intersecting relation to each other, opposed guide shoes disposed on opposite sides of center axes of the tapered barrel rolls and spearhead shaped plug disposed intermediate the tapered barrel rolls in front thereof. A round billet heated at temperature of 1150 to 1250.degree. C. is supplied to the hot rolling apparatus of the tapered roll type. The round billet in hot pierced at its center by the plug while it is being rotated by the tapered barrel rolls. Thereafter, the pierced billet is rolled repeatedly and formed into a seamless steel pipe. In this case, during the fabrication of the pipe, it assumes an elliptical shape due to compressive force and projective force exerted by the tapered barrel rolls. The guide shoes are arranged 90 degrees circumferentially of each roll in opposed relation to each other so as to control the outer shape and the thickness of the pipe. Therefore, the guide shoes are in contact with the steel pipe heated at elevated temperatures, so that the surface of the guide shoes are held in sliding contact with the rotatingly advancing steel pipes.
As a result, the guide shoes are repeatedly subjected to a rapid heating at elevated temperatures and a rapid cooling by cooling water. Further, the guide shoes undergo rolling sliding friction under greated stress load.
The guide shoes conventionally used under such serve conditions are made of a material such as an alloy consisting of 26% by weight of chromium--3% by weight of nickel--the balance iron alloy, 26% by weight of chromium--2% by weight of nickel--the balance iron alloy having thermal and wear resistant steel alloy at elevated temperatures, 1% by weight of carbon 5% by weight of copper--the balance iron alloy and 1% by weight of carbon--15% by weight of chromium--5% by weight of molybdenum--the balance nickel alloy. Some of these alloys affect a yield to fabricate a seamless steel pipe because of insufficient corrosion resistance at elevated temperatures. Scales or steel pieces formed at the surface of the steel pipe heated at elevated temperatures are stuck to the surface of the guide shoes by the heat involved. The stuck scales or steel pieces of the guide shoes give rise to damage to the surface thereby affecting the yield rate of the fabrication of the steel pipe. Also some of conventional alloys cannot withstand a thermal shock due to repeated of local heating and water cooling. As a result, cracks are formed on the surface of the guide shoe, so that subjected to damage.
Further some of these conventional alloys are not sufficient in wear resistance. Guide shoe made of such alloy has a shorter service life.
After an extensive study to provide an alloy which are sufficient in thermal resistance, wear resistance, toughness and hardness for use as guide shoes for a hot rolling apparatus of the tapered roller type for fabricating seamless steel pipe, this invention is achieved.